IV 



VERMES— ONTOGENY OF THE WOEMfi 



273 



head nepliridia of the Trocliopliora lies on each side. Tliese become tlie permanent 

 nephridia of tlie adult Fhoronis. The Actinotroclm thug formed sinks to the bottom 

 of tlie sea ; the invaginated trunk protrudes and grows quickly, the mid-gut at the 

 same time entering it and forming a loop with ascending and descending limbs. 

 The whole prostomium, with the neural plate and the larval tentacles, are thrown 

 ort' and devoured by the young Fhoronis. Through all these processes the body 

 has approached the adult stage ; it is quite evident that by the protrusion and 

 rapid gi-owth of the trunk, and the comparatively slight growth of the rest of the 

 original larval body, the anus comes to lie dorsally near the mouth. This process 

 readily allows of being referred back to the similar process in Sqmnculus, only there 

 the rudiment of the trunk is never invaginated into the larval body. 



BracMopoda. — The free-swimming larva oi Argio])e (Fig. 183) consists of three 

 consecutive divisions, which are called the anterior, middle, and posterior segments. 



Fig. 183.—^, free -swimming, B, 

 attaclied larva of Argiope, from 

 above (after Kowalewsky). vs. An- 

 terior segment ; ins, middle segment ; 

 hs, posterior segment ; m, mantle ; 

 rod, mid-gut ; wfc, ciliated organ. 



Fic. 184.— Vertical median 

 longitudinal section througli 

 an advanced embryo of Lin- 

 gula, after Brooks. >u, Mantle 

 folds : s'i, dorsal, sv, ventral 

 shell ; t, tentacles ; mli, man- 

 tle cavity ; Z/t, body cavity ; 

 St, stomodeeuni ; o, mouth ; 

 md, mid-gut ; M, hind-gut : vi 

 (below), shell muscles. 



The anterior segment is umbrella - shaped, and carries anteriorly 4 eyes. The 

 margin of the umbrella has longer cilia than the rest of the body. The middle 

 setnnent has dorsal and ventral folds directed posteriorly, and covering the posterior 

 segment. At each side of the free edge of the ventral fold two bundles of setse are 

 VOL. I T 



